The present invention is intended to be employed in a dialysis machine for measuring and/or monitoring the degree of ultrafiltration taking place during the dialysis procedure. A dialysis machine in which the invention can be employed is disclosed in European Patent No. B1-0,278,100 which describes a dialysis machine corresponding essentially to the commercial dialysis machine sold under the trademark GAMBRO AK 100.
Such a dialysis machine is provided with an inlet for an incoming liquid flow, such as pure water, and one or more inlets for the addition of concentrate thereto. The liquids from these inlets are mixed to form a dialysis solution, called the dialysate, which is supplied to a dialyser.
The dialyser includes a membrane which divides the dialyser into a blood side and a dialysate side. The dialysis solution passes over the dialysate side and purifies the blood by transporting impurities such as urea from the blood through the membrane to the dialysis solution, and by transporting necessary substances such as bicarbonate from the dialysis solution to the blood. In addition, a quantity of the liquid content in the blood, i.e., blood plasma, is drawn from the blood to the dialysis solution, as the so-called ultrafiltrate.
The dialysis solution is returned to the dialysis machine and discharged as an outgoing liquid flow from the dialysis machine to a drain.
Furthermore, the dialysis machine comprises a variety of means for regulating the composition of the dialysis solution, for achieving sufficient pressure and flow conditions for the dialysis solution, as well as for regulating and transporting the blood flow on the blood side of the dialyser. Thus, the dialysis machine includes a flow measuring device for measuring the flow of dialysis solution to and from the dialyser, and for calculating the ultrafiltrate as a difference between these flows.
German Patent No. A1-4,127,675 discloses a method and apparatus for monitoring the fluid flow in a conduit. A heat impulse is supplied to the fluid and the increase of temperature due to the supplied heat energy is used as a measurement of the mass flow through the conduit. The principle is called a thermal flow meter.
U.S. Pat. No. 4,530,759 discloses a method and apparatus for measuring the ultrafiltrate in a dialysis machine using a balancing device. The balancing device induces an error in the ultrafiltrate measurement due to different temperatures in the two balancing chambers, resulting in a small change of density. This error is compensated for by measuring the temperature difference and applying a correction factor thereto.
In a medical dialysis apparatus, it is customary to use a second independent measuring device for purposes of safety while monitoring significant properties and operating parameters of the dialysis machine.
The principal object of the present invention is to provide a device which monitors the ultrafiltrate totally independently of the normal operation of the dialysis machine.